The present invention relates to a high-frequency electron source, in particular in the form of an ion source neutralizer, in particular for an ion thruster, including a discharge chamber having at least one gas inlet for a gas to be ionized and at least one extraction opening for electrons.
In all applications where accelerated, electrically charged particles are needed—which is the case, for example, in surface treatment—ion beams must be neutralized after acceleration. Thus, aerospace engineers increasingly use electric propulsion units to propel satellites or space probes after they separate from the carrier rockets. Electric propulsion units are already being used today, especially for station-keeping of geostationary communications satellites. Ion propulsion units and SPT plasma propulsion units are mainly used for this purpose. Both types generate their thrust by ejecting accelerated ions. However, the ion beam must be neutralized to avoid charging the satellite.
The electrons needed to do this are provided from an electron source and incorporated into the ion beam through plasma coupling.
Up to now, aerospace engineers have used hollow-cathode plasma bridge neutralizers having electron emitters to neutralize these electric propulsion units (ion propulsion units and SPT plasma propulsion units). The neutralizer includes a cathode tube, which is terminated in the flow direction by a cathode disk having a central hole, and an anode disk that also has a central hole. An electron emitter, whose porous material is permeated by alkaline earth metals, including barium, is located inside the cathode tube. A coil-shaped electric heating element that heats the cathode tube and electron emitter is mounted on the outside of the cathode tube. The barium contained in the electron emitter emits electrons. A voltage applied between the anode disk and cathode disk accelerates these electrons. When a neutral gas, such as xenon, passes through the cathode tube, the electrons collide with the neutral gas atoms and ionize them, forming a plasma that is discharged through the hole in the anode disk.
A disadvantage of this system is that the emitter material contained in the electron emitter is hygroscopic and also reacts with oxygen at elevated temperatures. Consequently, this greatly limits its ability to be stored before installation, during mounting on the satellite and during commissioning prior to space launch. A further disadvantage of such complex and short-lived electron sources is that the emitter must be preheated for several minutes prior to activation.
An ion source neutralizer that includes a plasma chamber having walls made of a dielectric material and surrounded by a high-frequency coil is also known from U.S. Pat. No. 5,198,718.
A high-frequency electron source of this type generates electrons through a plasma that is produced through induction and maintained by a magnetic alternating field. This field is created by the high-frequency coil through which a high-frequency current flows. The electrons present in the plasma are accelerated by induction to speeds that, upon collision with a neutral atom in the plasma, can cause ionization thereof. During ionization, one or more further electrons are detached from the neutral atom, producing a continuous electron flow in the working gas jet.
The disadvantage of an electron source of this type is that a large portion of the energy needed to maintain the plasma in the plasma chamber is lost by the high-energy electrons from the plasma striking the chamber wall and thus being rebound to atoms. Through this process, not only are these electrons lost, but a large portion of the energy gained by the electrons through the alternating field is also dissipated. In addition, the high-frequency coil in the plasma chamber wall induces a ring current (eddy current), causing loss of energy that cannot be discharged to the plasma.